1. Field of Invention
The present invention relates to a current supply of a charger. More specifically, the present invention relates to a constant current (CC), constant voltage (CV) and constant temperature (CV) current supply of a rechargeable battery charger.
2. Description of Related Art
Batteries are used in a wide variety of portable devices such as laptop computers, cellular telephones, personal digital assistants (PDAs), radios, radiophones, stereo cassette tape players, etc. Two types of batteries are available: rechargeable and non-rechargeable. They exhibit different end of life voltage characteristics and effective series resistances. Non-rechargeable battery, e.g., alkaline battery, should not be subjected to recharging attempts. Rechargeable batteries, e.g., nickel-cadmium (Ni—Cd), nickel-hydrogen (Ni—H), lithium-ion, and nickel metal-hydride (Ni—MH) batteries, should be charged at different rates using different conditions.
These electronic devices powered by the rechargeable battery are typically plugged into the battery chargers when the rechargeable battery is in a low charge state.
FIG. 1 shows operation condition of a charger. Under Constant Voltage (CV) mode, the battery voltage is near the final voltage; and the charging current is close to 0. Under Constant Current (CC) mode, the battery voltage drops much below the final voltage; and the charging current equals preset value. Further, under constant temperature mode, the power dissipated by the charger is controlled such that the temperature inside the charger is kept constant.
FIG. 2 and FIG. 3 show prior control circuits for a conventional charger. The control circuit in FIG. 2 at least includes a resistor 110, voltage-controlled current sources 120 and 130, amplifiers 140 and 150 and a priority circuit 160.
As shown in FIG. 2, the priority circuit 160 has two inputs, A and B, and an output, Out. The priority circuit 160 connects the lower of the two inputs, A or B, to the output, Out. Therefore, the current flowing in the voltage-controlled current source 120 and in voltage-controlled current source 130 is equal to either the current required to develop V1 across the resistor 110 or the current required to develop V2 across the load Z1, whichever current is lower in magnitude. Voltage node, V3, also preferably continuously provides information regarding the magnitude of the charging current at all times. The higher magnitude current may be selected to charge the load.
The operating conditions of the control circuit in FIG. 2 are as follows. The load is a discharged battery, “V1/R110” (R110 refer to the resistance of the resistor 110) is equal to the desired charging current, and V2 is equal to the desired final float potential of the battery. When charging begins, V4, the voltage across the battery, is much lower than V2, and the output of amplifier 150 slews to the positive supply rail because amplifier 150 is requesting maximum current.
Then, the priority circuit 160 connects the output of amplifier 140 to the control voltage of the current sources and ignores the output of amplifier 150. Then, the current V1/R110 is delivered to the load Z1.
As the battery is charged and V4 approaches V2, the output of the amplifier 150 begins to drop. When the battery voltage V4 reaches V2, the current required by the load to maintain this voltage begins to drop below V1/R110. Amplifier 140 tries to force the current V1/R110 into the battery, but this causes V4 to rise above V2 which causes the output of amplifier 150 to fall quickly. The drop in the output of amplifier 150 causes the priority circuit 160 to choose the output of amplifier 150 as the controlling voltage for the current sources. At this point, the output of amplifier 140 is ignored. The current required by the load to maintain V4=V2 is less than the current V1/R110, so the voltage across resistor 110, labeled V3, falls below V1 and the output of amplifier 140 slews to the positive rail, and the priority circuit 160 continues to select the constant voltage loop to provide current to the load Z1. In summary, the current delivered to the load Z1 is preferably equal to V1/R110 until the voltage across the load Z1 reaches about V2. Then, the current delivered to the load Z1 is reduced in order to maintain V2 across the load Z1. This completes the constant-current/constant-voltage charging cycle.
Now please refer to FIG. 3. PMOS transistors 210 and 220 function as the voltage controlled current sources. Two diodes 230 and 240 and a pull-down current source 250 perform a diode- or function to implement the priority function performed by the priority circuit 160 in FIG. 2.
Circuit shown in FIG. 3 operates as follows. The gates of PMOS transistors 210 and 220 are coupled to the diodes 230 and 240. In addition, it is well known in the art that increasing gate voltage of a PMOS transistor, while holding the source fixed, decreases the drain-source current of a PMOS transistor. It follows that, whereas voltage-controlled current sources 120 and 130 provide higher current in response to a higher voltage, PMOS transistors 210 and 220 provide lower current in response to higher voltage. Furthermore, amplifiers 140 and 150 in FIG. 3 are connected in opposite polarity from the amplifiers 140 and 150 shown in FIG. 2.
In the constant current phase of circuit in FIG. 3, when the voltage across the load Z1 is less than V2, amplifier 140 sets the current V1/R110 to the load. The output of amplifier 140 is preferably the voltage required to force the non-inverting input of amplifier 140 to have a voltage V3 equal to V1. During this constant current phase of the circuit, the output of amplifier 150 is at the negative rail voltage. This negative rail voltage at the output of amplifier 150 is prevented from affecting the gate voltage of PMOS transistors 210 and 220 by diode 240. Therefore, the output of amplifier 140 controls the current to the load during this phase.
In the constant voltage phase of the circuit in FIG. 3, when the voltage across the load is at or above V2, amplifier 150 sets the current to the load such that this current is preferably less than V1/R110. During this constant voltage phase of the circuit, the output of amplifier 140 is at the negative rail voltage. This negative rail voltage at the output of amplifier 140 is prevented from affecting the gate voltage of PMOS transistors 210 and 220 by diode 230. Therefore, the output of amplifier 150 controls the current to the load during this phase.
It has been shown that, whichever output voltage from amplifiers 140 and 150 is higher, it controls the charging current to the load. Thus, one function of diodes 230 and 240 and PMOS transistors 210 and 220 is to select the higher output value of amplifiers 140 and 150 to provide the lower available or requested current to the load. Pull down current source 250 sets the base-line voltage of the gates of PMOS transistors 210 and 220 to zero so the higher output of the amplifiers can be used to accurately set the voltage of the gates.
Optimally, V1 is equal to V3 in the constant current phase. However, due to the limited gain of amplifier 140, an offset voltage Δ1 exists between V1 and V3 for the amplifier 140 to generate its output. If amplifier 140 is designed with zero systematic offset, the offset voltage Δ1 mainly comes from the mismatching of driving capabilities between diode 230 and current source 250. Since diode 230 and current source 250 have different control signals, their driving capabilities are not generally well matched. Process variation can worsen the mismatch between diode 230 and current source 250, thus increases the offset voltage Δ1. What's worse is that, in most cases, resistor 110 is an external component outside the chip, and the pole created by the resistor 110 and parasitic capacitor at node V3 prevents the using of high gain amplifier for 140 for fear of instability. Low gain for amplifier 140 can result in high offset voltage Δ1. Therefore, the charge current into the load (i.e. the battery) is not precisely enough.
Optimally, V2 is equal to V4 in the constant voltage phase. However, due to the limited gain of amplifier 150, an offset voltage Δ2 exists between V2 and V4 for the amplifier 150 to generate its output. If amplifier 150 is designed with zero systematic offset, the offset voltage Δ2 mainly comes from the mismatching of driving capabilities between diode 240 and current source 250. Since diode 240 and current source 250 have different control signals, their driving capabilities are not generally well matched. Process variation can worsen the mismatch between diode 240 and current source 250, and thus increases the offset voltage Δ2 and deteriorates the accuracy of the final voltage of the battery.